The present invention is related to the field of battery protection circuits.
It is common to incorporate protection circuitry in battery-based power supplies. Protection circuitry detects certain battery conditions, and in response the circuitry controls the charging and discharging of the battery to prevent unsafe operation or operation that can damage the battery. Certain types of batteries, such as lithium-ion batteries, require protection from conditions such as overheating, overcharging, and over-discharging to prevent potentially explosive failure.
Generally, battery protection circuits include detection circuitry that monitors battery voltage, current, or other operating parameters, and one or more controlled elements such as transistors that interrupt or otherwise control battery current when an unsafe condition is detected.
One protection circuit in common use today includes two metal-oxide-semiconductor field-effect transistors (MOSFETs) arranged in a back-to-back configuration in series between the battery and a connection point for the load and charger. The respective gate voltages of the MOSFET are controlled by signals from detection circuitry that detects overcharged and over-discharged conditions of the battery. When both transistors are conducting, either charge or discharge current can flow. When either transistor is off, current is prevented from flowing in one direction. A parasitic element associated with each transistor, which is referred to as a "body diode", allows current to flow in the other direction. As a result, when the battery is in an overcharged condition, for example, the body diode of the non-conducting transistor permits the flow of discharge current but prevents the flow of charge current. Similar operation obtains when the battery is in an over-discharged condition.
The above approach suffers from the problem of excessive power dissipation under both overcharged and over-discharged operating conditions. Because current is flowing through a forward-biased diode with a non-negligible voltage drop, the power dissipated in the transistor is considerable. For example, if the charge or discharge current is 1 ampere, dissipated power is on the order of 1 watt. For many applications, such wasteful operation is very undesirable.